Arctic sea-ice loss is a leading indicator of climate change and can be attributed, in large
part, to atmospheric forcing. Here, we show that recent ice reductions, weakening of the
halocline, and shoaling of the intermediate-depth Atlantic Water layer in the eastern
Eurasian Basin have increased winter ventilation in the ocean interior, making this region
structurally similar to that of the western Eurasian Basin. The associated enhanced
release of oceanic heat has reduced winter sea-ice formation at a rate now comparable to
losses from atmospheric thermodynamic forcing, thus explaining the recent reduction in
sea-ice cover in the eastern Eurasian Basin. This encroaching “atlantification” of the
Eurasian Basin represents an essential step toward a new Arctic climate state, with a
substantially greater role for Atlantic inflows.

Over the past decade, the Arctic Ocean has experienced dramatic sea-ice loss in the summers, with record-breaking years in 2007 and 2012 for both the Amerasian Basin and the Eurasian Basin (EB). More
remarkably, the eastern EB has been nearly ice-free (<10% ice coverage) at the end of summer
since 2011 (Fig. 1). Most sea-ice-mass loss results
from summer solar heating of the surface mixed
layer (SML) through cracks in the ice and open
water, and consequent melting of the lower surface of the ice (1–3). Heat advected into the EB
interior by Atlantic water (AW) generally has not
been considered an important contributor to sea-ice reduction, due to effective insulation of the
overlying cold halocline layer (CHL) (4) that separates the cold and fresh SML and pack ice from
heat carried by the warm and saline AW.

There are, however, reasons to believe the role
of AW heat in sea-ice reduction is not negligible
and may be increasingly important (5). Nansen
(6) identified the importance of warm (
temperature >0°C) and salty intermediate-depth (150
to 900 m) AW in establishing the thermal state
of the Arctic Ocean. Later studies demonstrated
that AW is transported cyclonically (
counterclockwise) along the deep Arctic Basin margins (7–10),

carrying enough heat, if released, to melt the Arctic
sea ice many times over. Observations from the

1990s and 2000s documented two warm, pulse-like AW temperature anomalies on the order of 1°C
(relative to the 1970s), entering the Arctic through
Fram Strait and occupying large areas of the Arctic
Ocean (11–14). The strength of the 2000s warming
peaked in 2007–2008, with no analogy since the
1950s (14). This AW warming has slowed slightly
since 2008 (Fig. 2C).

Strong stratification, which is found in most ofthe Arctic Ocean, prevents vigorous ventilationof the AW. One notable exception is the westernNansen Basin, north and northeast of Svalbard,where proximity to the sources of inflowing AWmakes possible strong interactions between theSML and the ocean interior (5). Specifically, weaklystratified AW entering the Nansen Basin throughFram Strait is subject to direct ventilation inwinter, caused by cooling and haline convectionassociated with sea-ice formation (15). This venti-lation leads to the reduction of sea-ice thicknessalong the continental slope off Svalbard (16, 17).In the past, these conditions have been limited tothe western EB, because winter ventilation ofAW in the eastern EB was constrained by strongerstratification there. However, newly acquired datashow that conditions previously only identified inthe western Nansen Basin now can be observedin the eastern EB as well. We call this eastwardprogression of the western EB conditions the“atlantification” of the EB of the Arctic Ocean.

Overview of sea-ice state

The progressive decline in sea-ice coverage of the
Arctic Ocean during the satellite era, at 13.4% per
decade during September (18), has been accompanied by decreases in average sea-ice thickness
of at least 1.7 m in the central Arctic (19, 20). In the
region of the eastern EB defined by the polygon
in Fig. 1A, the local changes since 2003 have also
been substantial. With the northward retreat of
multiyear sea-ice cover (21), coverage within that
polygon is now dominated by seasonal ice, either
advected from the east and south or produced
locally. Mean September ice coverage has been
<10% of the total area during the past 5 years,
portending ice-free summers in coming years if
current sea-ice trends prevail. Annual open-water
coverage has increased from less than 1 month to
more than 3 months in recent years (Fig. 1B); these
longer ice-free periods, maintained by atmospheric
and ocean conditions, increase direct air-ocean interactions (momentum and energy exchanges).
Available satellite estimates of ice thickness in this
region—typically sparse—suggest a concurrent trend,
leading to an overall thinning of ~0.5 m (in March)
from 2003 through 2015 (Fig. 1C). Satellite records
show that this pattern continued in 2016, with
less extensive (compared with record minimum)
December sea-ice extent in the Kara and Barents
Seas (22).

Role of atmospheric thermodynamics in
sea-ice decline

Arctic-wide warming is evident from surface air
temperature trends ranging between 0.1° and 0.3°C
per decade for the period 1984 to 2012 (23). Surface
air temperature trends from weather stations and
ERA-Interim reanalysis data for the Laptev Sea
and eastern EB region far exceed observed average
Arctic regional trends (fig. S1, A to C), consistent
with recently enhanced sea-ice decline. The net atmospheric thermodynamic effect on sea ice cannot be quantified using surface air temperature
records alone, because changes in this parameter
omit thermodynamic forcing due to additional atmospheric processes.

Fortunately, records are available for fast-ice (
motionless seasonal sea ice anchored to the shore,
which melts and refreezes each year) thickness,
providing a measure of nearly pure atmospheric
thermodynamic forcing over the broad, shallow
Siberian shelves, where the effect of advected or
seasonally stored oceanic heat is negligible. Records